This invention relates generally to precipitation methods for decontaminating various types of solutions which are contaminated with a variety of contaminants such as heavy metals and radioactive compounds, using a novel combination of treatment steps. More particularly, this invention relates to methods for remediating water contaminated with uranium, thorium, mercury and/or copper, using sodium silicate, ammonium hydroxide and hydrochloric acid, to precipitate the contaminants and ultimately separate them from solution.
There is increasing concern over the hazards posed by the rising levels of inorganic contaminants within the world's water supplies due to accidental spills, leaks, mining practices and poor disposal practices. Most heavy metal contaminants are toxic to some degree to all life-forms. In humans, toxic heavy metal poisoning can lead to severe nervous system disorders and can cause death.
It has been suggested that various inorganic contaminants in solution can be removed via precipitation solution using, for example, carbonates, hydroxides, sulfides, and/or silicates. Such techniques are described in Canter, L. W., and Knox, R. C., Ground Water Pollution Control, Lewis Publishers, Inc., 1985, pp. 110-120; and Willey, B. R., Finding Treatment Options for Inorganics, in WATER/Engineering & Management, October, 1987, pp. 28-31.
In particular, the use of sodium silicate (also referred to as water glass or Na.sub.2 SiO.sub.3) to remove uranium and/or thorium from waste streams has been suggested. For example, in U.S. Pat. No. 4,501,691, issued in the name of Tanaka et al., on Feb. 26, 1985, there is described a process for treating radioactive liquid waste in which the liquid waste is treated with sodium silicate to form a uranium containing silica precipitate. The precipitate subsequently is treated in a step-wise fashion with acid to recover the uranium, and then alkali metal hydroxide solution to regenerate the water glass.
Silicate precipitation processes for uranium and thorium recovery also are described in U.S. Pat. Nos. 4,349,513, issued Sep. 14, 1982, in the name of Ishiwata et al.; U.S. Pat. No. 5,077,020, issued Dec. 31, 1991, in the name of Lahoda et al; and U.S. Pat. No. 4,338,286, issued Jul. 6, 1982, in the name of Nakai et al. In Ishiwata, the liquid waste is treated with sodium silicate in the presence of aqueous fluorine and ammonia to make a uranium/thorium-containing silicate precipitate which is filtered out of the process stream find sent to a holding tank. In Lahoda et al., the contaminated stream is treated with sodium silicate in the presence of ammonia, fluoride, and nitrate in water. Nakai et al. generally describe a precipitation process for treating liquids contaminated with uranium/thorium wherein sodium silicate is added to the solution in the presence of ammonia water and chlorine to cause a contaminant-containing silica precipitate to form.
There are significant disadvantages associated with the application of each of these methods. For example, carbonate systems, while relatively easy to operate, are difficult to control and often result in processing problems such as premature plugging of equipment. Sulfide systems are difficult to handle, complex to operate, and frequently produce a high waste volume and harmful residual levels of precipitating agent. Hydroxide systems are widely used to remove inorganics because they are the most reliable, and have the added advantages of ease in chemical handling and low volume of sludge. However, the resulting sludge often is gelatinous and difficult to dewater, making treatment, separation, and storage of the contaminated material difficult. In addition, pH must be precisely controlled. Otherwise, contaminant-containing precipitate can readily go back into solution.
The above mentioned silicate precipitation methods suffer from at least two critical drawbacks. First, typically such methods produce a contaminant-containing, slime or sludge which is not readily treatable, separable, or easily stored. For example, the slime/sludge is not easily dewatered, making further treatment with filtration devices either impossible (due to plugging) or impractical (due to excessively slow filtration rates). Second, silicate precipitation generally is not effective on other inorganic contaminants; for example, silicates do not readily precipitate other heavy metals like mercury. Consequently, silicate precipitation methods are slow, inefficient, and ineffective in reducing the level of uranium, thorium and other heavy metals to environmentally acceptable levels.
What is needed is a simplified, easy-to-operate method of treating large volumes of solutions containing heavy metals and radioactive contaminants, singly or in combination, which effectively segregates the contaminates from the clean solution and concentrates the contaminated material in a manageable, low volume, concentrated waste stream.
There is a further need for a system that can effectively and economically remove metals from contaminated solutions, whereby the contaminated material is readily separable from the cleansed solution, especially by filtration methods.
There is also a need for a process which can effectively remove various metal contaminants like uranium, thorium, mercury, and copper from solution.